Previous rotational systems for rotating large parabolic trough solar collectors to allow them to track the movement of the sun across the sky used a variety of equipment configurations including: (1) motorized gearboxes (e.g. worm gear drive units) to provide the slow-movement, high-torque output needed for precise sun tracking (for example, the Luz Solar-1 (LS-1) and Luz Solar-2 (LS-2) solar collectors of the California Luz company installed at the large solar electric generation station (SEGS) systems in California used worm gear drive units); (2) hydraulic cylinders (which elongate in a linear fashion when pressurized by hydraulic fluid) arranged in a mechanical system that converts the linear motion of the cylinders into the desired rotational motion; and (3) rack-and-pinion rotary actuators (that provide for rotation during sunny normal-operating conditions) along with a companion mechanical locking system that provides enough added strength to resist the very high torques produced by the solar collectors during high wind conditions.
However, these prior systems had a number of disadvantages. For example motorized gearboxes (used in the early SEGS systems installed in the mid-to-late 1980s in California) had two main disadvantages. First, these high-reduction systems use worm gearing, a gear arrangement in which a worm, which is a gear in the form of a screw, meshes with a worm gear. This approach is imprecise owing to significant “backlash” inherent in worm gearing. Second, the contact area between the worm and the worm gear is quite small. This small contact area results in extremely limited overload capability. To raise the overload capability, the size of the gearing must be increased substantially, which substantially increases costs.
Hydraulic cylinders can be more precise than gearboxes, since they do not have backlash problems. The rod of the hydraulic cylinder extends (or contracts) as pressurized fluid is pumped into one side (or the other) of the piston inside the cylinder. But a hydraulic cylinder is a linear device, and even when arranged as a mechanical linkage it cannot provide the desired rotation range of 180 to 240 degrees (and generally is not used when a rotation range over 120 degrees is needed) owing to physical limitations of linkages. So, to achieve the rotation range that is needed by parabolic trough collectors, two linear cylinders are used in combination. This results in a complicated mechanical system, which is expensive. This kind of complicated system has been used on systems in the U.S. (the SEGS 8 and 9 solar projects in Harper Lake, Calif., as well as some projects in Europe).
Hydraulic rack and pinion rotary actuators (for example as used at a recently completed parabolic trough system in Nevada) do indeed directly provide rotational output, but these rack-and-pinion actuators have very limited rotational overload capability. This limited overload capability is the result of the small amount of contact area between the pinion gear and the gear rack. The high forces produced during high wind events, when reacted to by the small amount of contact area between the pinion gear and the gear rack, result in gear contact pressures that are too high for the gearing to withstand. In practice, to compensate for the very limited overload capability of the rack and pinion gearing, another external locking device is added that can handle these high forces, which eliminates the need for the rack-and-pinion gearing to handle these very high contact pressures. This external locking device adds significant cost, adds complexity, and has been found to be difficult to employ with reliability in actual field operations.
The following U.S. patents are believed relevant to aspects of this disclosure: U.S. Pat. Nos. 3,511,134, 3,548,866, 3,559,534, 4,161,905, 4,328,789, 4,628,692, 4,739,620, 5,138,838, 5,531,216, 5,806,553, 6,123,067, 6,662,801, and 6,772,671. All patents and publications referred to herein and all patents and publications submitted herewith, are incorporated herein by reference to the extent not inconsistent herewith for purposes of written description and enablement.
The foregoing examples of the related art and its limitations are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.